CHAPTER 29 DNA: Genetic Information, Recombination, and Mutation to accompany Biochemistry, 2/e by Reginald Garrett and Charles Grisham All rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887-6777
Outline • 29.1 DNA Carries Genetic Information • 29.2 Genetic Information in Bacteria • 29.3 Molecular Mechanism of Recombination • 29.4 The Immunoglobulin Genes • 29.5 Molecular Nature of Mutation • 29.6 RNA as Genetic Material • 29.7 Transgenic Animals
The Transforming Principle is DNA • Griffith in 1928 injected bacteria into mice • Combination of heat-killed type S and non-virulent type R killed the mice • Showed that type S had been transformed by the type R! • In 1944, Oswald Avery showed that the active transforming agent was DNA!
Further Proof • In 1952, Hershey and Chase, studying bacteriophages, labelled DNA with 32P and protein with 35S • Bacteriophage progeny produced by infection of bacteria contained 32P (thus DNA from the original phage), but not 35S (from the protein)!
Genetic Information Can Be Transferred Between Bacteria • In 1946, Lederberg and Tatum showed that two different strains of bacteria with different growth requirements could exchange genes • Lederberg and Tatum surmised that the bacterial cells must interact with each other - the process is now known as sexual conjugation
Chromosome Mappingby Interrupted Mating • Passage of the ‘F factor’ from Hfr cells to F- cells also brings adjacent genes • Genes from the Hfr chromosome are transferred in a fixed order • This order can be mapped by ‘interrupted mating’ methods • The genetic map obtained reveals a circular arrangement of genes
Mechanism of Recombination • General recombination: any pair of homologous DNA segments as substrates • In 1964, Robin Holliday proposed a model involving single-stranded nicks at homologous sites • Duplex unwinding, strand invasion and ligation create a Holliday junction
Enzymology of Recombination • RecBCD initiates recombination in E.coli • RecA forms nucleoprotein filament for strand invasion and homologous pairing • RuvA, RuvB, RuvC drive branch migration and process the Holliday junction into recombination products • Eukaryotic systems are probably similar
The RecA Protein • 38 kD enzyme that catalyzes ATP-dependent DNA strand exchange, leading to formation of Holliday junction • RecA forms a helical filament with a groove to accommodate DNA • RecA:ssDNA complex binds dsDNA at secondary site and searches for regions homologous with the bound ssDNA, then forms the desired duplex
Resolving Holliday Junctions • Ruv proteins resolve the junction into recombination products • RuvA and RuvB act as a helicase that dissociates the RecA filament and catalyzes branch migration • RuvC is an endonuclease that binds at the junction and cuts pairs of DNA strands of similar polarity. Splice and patch recombinants result.
Transposons • In 1950, Barbara McClintock showed that activator genes in corn could move freely about the genome. • This was at first viewed as heresy and McClintock was ostracized • Molecular biologists in the late 1970s rediscovered what McClintock knew • She received a MacArthur Award in 1981 and a Nobel in 1983
Immunoglobulin Genes • Immunoglobulin genes maximize protein diversity with a limited amount of genetic information • DNA rearrangement (genetic recombination) during assembly of IG genes accounts for this diversity • Three separate genes are combined to form the L-chain gene • Four genes combine to form H-gene
Molecular Nature of Mutation • Point mutations - one base for another - arise from mispairing, insertion of analogs or chemical mutagens • Mutagens include nitrous acid, hydroxylamine and alkylating agents • Insertions and deletions result in frameshift mutations
PrionsProteins as Genetic Agents? • Prions are ‘protein infectious particles’ • Agents composed only of protein • Responsible for kuru, Creutzfeld-Jacob disease, mad-cow disease, etc. • The ‘infection’ appears to involve a change of secondary structure and conformation (or conformations!) in the prion protein • A Nobel for Stanley Prusiner in 1997
Transgenic Animals • Genes can be introduced into animals by transfection - injection of plasmid DNA into recipient cells • Plasmids can be injected into fertilized eggs in mice • Expression is usually variable, because the gene is inserted randomly • Growth hormone transfection produces mice that are very large!